Support timezones

This patch adds support for timezones in date formats.

```haskell
parseTime defaultTimeLocale "%Y-%m-%dT%H:%M:%S%Z" "2000-02-02T00:00:00+0100" :: Maybe UTCTime
-- Just 2000-02-01 23:00:00 UTC
parseTime defaultTimeLocale "%Y-%m-%dT%H:%M:%S%Z" "2000-02-02T00:00:00Z" :: Maybe UTCTime
-- Just 2000-02-02 00:00:00 UTC
parseTime defaultTimeLocale "%Y-%m-%d %H:%M:%S%Z" "2000-02-02 00:00:00" :: Maybe UTCTime
-- Just 2000-02-02 00:00:00 UTC
```